Add CentOS/RHEL 6 to GitLab CI

We need something really old, but support in out GitLab CI. CentOS 6 (+EPEL 6) seems like a good fit.

Rework IDN support in dig #384 (closed)
Fix a Net::DNS version quirk in the "resolver" system test #393 (closed)
Do not use Net::DNS::Nameserver in the "serve-stale" system test #399 (closed)

Edited Jul 10, 2018 by Michał Kępień